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XMM-Newton has produced an extensive X-ray source catalogue in which thestandard pipeline determines the variability of sufficiently bright sourcesthrough chi-square and fractional variability tests. Faint sources, however,are not automatically checked for variability, thus overlooking faint, shorttimescale transients. Our goal is to find new faint, fast transients inXMM-Newton EPIC-pn observations. To that end we have created the EPIC-pnXMM-Newton Outburst Detector (EXOD) algorithm, which we run on the EPIC-pn dataavailable in the 3XMM-DR8 catalogue. In EXOD, we compute the whole-fieldvariability by binning in time the counts in each detector pixel. We nextcompute the maximum-to-median count difference in each pixel to detectvariability. We applied EXOD to 5,751 observations and compared the variabilityof the detected sources to the standard chi-square and Kolmogorov-Smirnov (KS)variability tests. The algorithm is able to detect periodic and aperiodicvariability, short and long flares. Of the sources detected by EXOD, 60-95% arealso shown to be variable by the chi-square and KS tests. We obtain a netnumber of 2,536 variable sources. Of these we investigate the nature of 35sources with no previously confirmed classification. Amongst the new sources,we find stellar flares and AGNs; but also four extragalactic type I X-raybursters that double the known neutron-star population in M31. This algorithmis a powerful tool to promptly detect variable sources in XMM-Newtonobservations. EXOD detects fast transients that other variability testsclassify as non-variable due to their short duration and low number of counts.Finally, EXOD allows us to detect and identify the nature of rare compactobjects through their variability. We demonstrate this through the discovery offour extragalactic neutron-star low mass X-ray binaries, doubling the number ofknown neutron stars in M31.

The EXOD search for faint transients in XMM-Newton observations: Method and discovery of four extragalactic Type I X-ray Bursters